Added op:join to concat objects with separator between elements.
Merge pull request #607 from Netflix/docs

Javadoc improvements:
Merge pull request #604 from akarnokd/StringObservableOps1

Added op:join to concat objects with separator between elements.
Operation Timer 3.0
CustomReplaySubject to support various replay() operators.
Operation: Replay additional overloads
Merge pull request #609 from akarnokd/OperationTimer3

Operation Timer 3.0
Merge pull request #612 from akarnokd/ReplayAdditionalOps

Replay additional overloads
Copied the code from OperationMerge
Merge pull request #615 from abersnaze/issue614

Copied the code from OperationMerge to fix synchronization
replay() and timer() javadocs: diagrams & wiki links